This invention relates to a spot-weldable aluminum or aluminum alloy sheet which can be spot-welded with a decreased welding current over an increased number of weldable spots in continuous spot welding.
Because of a low specific weight of 2.7 (which is about one-third that of iron), aluminum sheet (which includes aluminum alloy sheet) has begun to be employed in some automobiles to constitute the whole bodies or parts of the bodies such as automobile hoods, for the purposes of saving weight and thereby reducing fuel consumption.
A major problem involved in the use of aluminum sheet in such applications is attributable to its spot weldability, which is significantly inferior to that of steel sheet conventionally used for automobile bodies.
Compared to steel, aluminum has significantly poorer heat generation efficiency in resistance welding such as spot welding since it is difficult to generate heat due to its low electrical resistivity, which is on the order of one-third to one-fourth that of steel, and the generated heat easily escapes due to its high thermal conductivity, which is on the order of 2 to 3 times that of steel. As a result, spot welding of aluminum sheets requires a current which is about four times as large as that required for spot welding of steel sheet. The following Table 1 shows the electrical and thermal conductivities of mild steel and 5000- and 6000-series aluminum alloys,
TABLE 1 ______________________________________ Electrical Thermal Conductivity.sup.1) Conductivity.sup.2) ______________________________________ Mild Steel 11-12% 0.13-0.11 5000-series Al alloy about 33% about 0.28 6000-series Al alloy about 45% about 0.45 ______________________________________ (Note): .sup.1) LACS%, .sup.2) cal/.degree.C.cm.sec (at 20.degree. C.)
Since a large current is passed in spot welding of aluminum sheets, as described above, between the aluminum sheets and the electrodes of a spot welder, which are usually made of Cu or a Cu alloy, the surface of the electrodes tends to be rapidly contaminated with aluminum to form a brittle Cu-Al alloy, As a result, the dressing life of the electrodes, which is expressed in terms of the number of weldable spots in continuous spot welding before dressing (regrinding) of the contaminated electrodes becomes necessary, is as small as between about 200 and about 300 spots, which is much smaller than the dressing life of 10,000 spots or more obtained with steel sheet, and this greatly interferes with the manufacture of automobile bodies from-aluminum sheet. The dressing life or the number of weldable spots is an indication of continuous spot weldability,
Thus, the spot weldability of aluminum sheet is significantly inferior to that of steel sheet, particularly with respect to the welding current: required and the number of weldable spots in continuous welding.
In Japanese Patent Application Laid-Open No. 53-6252(1978) it is described that spot welding of aluminum sheet can be performed by interposing a thin zinc film between two aluminum sheets to be spot welded. The thin zinc film is either a zinc foil inserted in the interface between the two sheets or a zinc coating or plating formed on one or both of the aluminum sheets.
According to that method, it is expected that the efficiency of heat generation can be improved by the zinc film interposed between the aluminum sheets, resulting in a decrease in welding current. However, the improvement attained by the method depends on the thickness of the zinc film, and a sufficient effect cannot be obtained with a zinc film having a thickness of about 10 .mu.m or less, which approximately corresponds to a weight of about 70 g/m.sup.2 or less. On the other hand, the interposition of a Zn film thicker than 10 .mu.m undesirably increases the manufacturing costs and decreases the adhesion of the film. When the film is a Zn plating, such a thick Zn plating on an aluminum sheet is often uneven and may cause flaking and powdering during press-forming of the plated aluminum sheet.
Furthermore, the above-described method does not modify the surface of the electrodes of a spot welder. Therefore, it cannot provide appreciable improvement in the continuous spot weldability (dressing life).